In recent years, with increase in traveling distances and increase in loading capacities on a global scale, there is a need for railway wheels (hereinafter, also called “wheels”) having longer life than the conventional wheels.
The damage factors on wheel tread mainly include three phenomena that are (i) wear, (ii) rolling contact fatigue and (iii) spalling, and especially in recent years, the number of wheels that are damaged by wear associated with increase in traveling distances and rolling contact fatigue associated with increase in loading capacities has been increasing. Rolling contact fatigue is sometimes called “shelling”. Though a crack that is caused by spalling is sometimes called “shelling”, the occurrence of the crack due to formation of a white layer is defined herein as “spalling”.
High temperature rolling contact fatigue (Thermal mechanical shelling, hereinafter called “TMS”) that occurs with rise in the wheel temperature during braking is currently said to be the cause of a wheel damage. With this, the wheels ensuring high temperature strength are required. For example, in the Class-D standard of AAR (Association of American Railroads), the yield strength at 538° C. (1000° F.) is defined to be 345 MPa or more.
Recently, in order to suppress crack occurrence on a wheel tread, it is required to ensure the minimum ductility, and different countries have different definitions. For example, in the Russia GOST10791 Grade 3 standard, the elongation is defined to be 8% or more, in the China TB/T 2708 CL60 definition, the elongation is defined to be 10% or more, in the Europe EN 13262 ER9 standard, the elongation is defined to be 12% or more, in the Class-D definition of AAR, the elongation is defined to be 14% or more, and the like.
It is empirically known that wear resistance and rolling contact fatigue resistance are properties contrary to spalling resistance. It is urgently required to develop the steel for wheel that is excellent in balance of wear resistance, rolling contact fatigue resistance and spalling resistance, further includes both high strength at high temperature and high ductility, and can give long life to wheels.
For example, the technology relating to wheels are disclosed in the following documents.
Patent Document 1 discloses “high toughness steel for railway wheel” adding V.
Patent Document 2 discloses “rim or monoblock wheel for a wheel set of a railway vehicle” that is excellent in wear resistance, fracture resistance and thermal crack resistance.
Patent Document 3 discloses “railway wheel” that makes shelling resistance and flat (spoiling) resistance as thermal crack resistance compatible by decreasing the content of C, and making the tread portion have a bainitic microstructure, a tempered martensitic microstructure, or a mixed microstructure of bainite and tempered martensite.
Patent Document 4 discloses “high carbon railway wheel excellent in wear resistance and thermal crack resistance” in which the content of C is increased to 0.85 to 1.20%.
Patent Document 5 discloses “railway wheel excellent in wear resistance and thermal crack resistance” that is an monoblock type railway wheel formed of steel having a chemical composition consist of C: 0.4 to 0.75%, Si: 0.4 to 0.95%, Mn: 0.6 to 1.2%, Cr: 0 to less than 0.2%, P: 0.03% or less and S: 0.03% or less, and the balance consisting of Fe and impurities, wherein the region up to at least the depth of 50 mm from the surface of the wheel tread consists of a pearlitic microstructure, and the manufacturing method thereof.
Patent Document 6 and Patent Document 7 disclose “steel for railway wheel” which are increased in strength by containing 0.01 to 0.12% and 0.009 to 0.013% of Nb respectively, and are improved in rolling contact fatigue resistance and spalling resistance.
Patent Document 8 discloses a wheel steel containing V or Nb. According to the invention, it is described that rolling contact fatigue resistance property can be ensured without thermal refining.
Patent Document 9 discloses high strength rail using steel with pearlite grains refined by Ti deoxidation. According to the invention, it is described that ductility and toughness can be improved.
Patent Document 10 discloses the material that is improved in rolling contact fatigue resistance property by defining the size of an alumina cluster.
Patent Document 11 discloses the wheel steel with high TMS resistance, which is improved in high temperature strength by increasing the contents of Si, Cr and Mo.